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How Planet Nine may have been exiled to solar system's edge

If the giant planet announced this week actually exists, we can already explain how it got there – and how it solves a mystery about the worlds we see around other stars

Caltech/R. Hurt (IPAC)

By Adam Becker

OUR solar system might not be the weird kid on the block after all. Astronomers announced on 20 January that they have strong evidence for a planet around 10 times Earth's mass lurking unseen in the outer solar system. If it bears out, this world would fill a hole in our solar system's family tree: a missing super-Earth.

Super-Earths, also known as mini-Neptunes since they may be gassy or rocky, are among the most common planets in the Milky Way. Hundreds have been spotted in other solar systems over the past 20 years, but there appeared to be none at home.

Now Konstantin Batygin and Michael Brown at the California Institute of Technology say they can make a convincing case for a super-Earth – dubbed Planet Nine – here in our solar system. They believe that the existence of such a planet would explain an oddity seen in the orbits of six objects found in the Kuiper belt (see diagram).

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“We usually talk about how the most common type of planet, two to three times the size of Earth, is not found in our solar system, yet now we might have one,” says Sara Seager at the Massachusetts Institute for Technology. “That's cool.”

The most common type of planet is not found in our solar system, yet now we might have one

But even if there is a super-Earth out on the edges of our solar system, we are still pretty weird. “Even with Planet Nine bringing us a bit closer to the galactic planetary catalogue, the solar system does still appear to be an overall outlier,” says Batygin, as super-Earths in other star systems are much closer to their stars.

Simulations of the young solar system suggest that the early formation of Jupiter probably prevented this from happening here. Those simulations also suggest that a super-Earth couldn't have formed as far away as Planet Nine would have to be – there simply wasn't enough material out there to make a planet that big.

So how did Planet Nine, if it exists, get all the way out there?

The answer must lie in the very beginnings of the solar system, when our stellar neighbourhood was a very different – and more dangerous – place. “We know the process of planet formation is not placid,” says Batygin. “There was a very chaotic sequence of events.” Planets were thrown around by their gravitational pulls on each other, occasionally colliding.

Batygin believes Planet Nine was the “core of a giant planet, which was forming with the other planets we know and love, that was thrown out of the solar system”.

Violent births

“We know that Uranus and Neptune probably had a very violent final phase of formation,” says Sean Raymond of Bordeaux Observatory in France – they essentially came together through a series of collisions. Last year, he and colleagues simulated this process and found that planets around five times the mass of Earth were ejected from the solar system during the pileup.

At the time the team didn't think too much about where these ended up, but hints of Planet Nine suggest one could have settled into a lengthy orbit. “This extra planet could be representative of the building blocks of the ice giants,” says Raymond.

That gives us a new mystery: what stopped the planet escaping from the solar system? Brown believes that gas and dust in the very early solar system may have slowed it down. Batygin has his eye on a different culprit: nearby stars. He thinks their gravitational tugs are the best candidates for slowing down Planet Nine enough to keep it in a huge, elongated orbit around the sun.

But Planet Nine might not even be there. While Batygin and Brown have compelling evidence for its existence, that is not the same as a discovery.

The true test will come when further objects in the Kuiper belt are found. If the anomalies remain, then Planet Nine will be the most likely explanation.

If Planet Nine really is out there, with any luck astronomers will be able to spot it soon, despite the scant light it reflects from the sun. “Finding this object is like finding a light bulb on the moon,” says Batygin. “But our calculations have provided a roadmap for where to look for this very dim object in the sky. Hopefully, that will trigger a hunt.”

We might even send a probe there. “It's not as crazy as it sounds,” says Brown. He suggests using the sun as a gravitational slingshot. Depending on how far away Planet Nine is, it could take anywhere up to 20 years to reach.

Similar schemes have been proposed in the past, some with nuclear-powered rockets, but they never got past the planning stages. A target like Planet Nine could make all the difference.

“The societal impact of having that destination would be very exciting,” says Greg Laughlin at the University of California, Santa Cruz. “The stars are just too far away, we're not going there. But this is a place we could get to.”

This article appeared in print under the headline “Exiled to the solar system's edge”